Recently, there has been an explosive increase in the demand for wireless data services. Further, an evolution from a wireless voice service to a wireless data service requires a gradual increase of wireless capacity. To cope with such a demand, wireless service providers and wireless equipment manufacturers attempt to improve a data rate of a wireless system, which results in motivating massive researches. A wireless channel experiences various problems such as path loss, shadowing, fading, noise, limited bandwidth, power limit of a user equipment (UE), and interference between other users. Due to these limitations, the wireless channel has a shape of a narrow pipe that obstructs fast data flow, and it is difficult to design an effective bandwidth of wireless communication providing high-speed data transmission. The designing of the wireless system has other challenges such as resource allocation, mobile issues in association with a rapidly changing physical channel, portability, security, and privacy.
When a transport channel experiences deep fading, if a different version or a replica of a transmitted signal is not additionally transmitted, it is difficult for a receiver to determine the transmitted signal. A resource corresponding to the different version or the replica is referred to as diversity, and is one of most important factors that contribute to reliable transmission through a wireless channel. The use of the diversity can maximize data transfer capacity or data transfer reliability. A system for implementing the diversity by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas is referred to as a multiple input multiple output (MIMO) system. The MIMO system is also referred to as a multiple antenna system.
Examples of the multi-antenna scheme include space frequency block coding (SFBC), space time block coding (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), generalized cyclic delay diversity (GCDD), selective virtual antenna permutation (S-VAP), etc. When the UE is initially synchronized to a base station (BS), the UE cannot know the number of Tx antennas of the BS. Therefore, the BS can use a transparent multi-antenna scheme (e.g., TSTD, PVS, CDD, etc) capable of receiving a synchronization signal even if the UE does not know the number of Tx antennas. However, according to the transparent multi-antenna scheme, the UE cannot estimate a channel for each Tx antenna of the BS, and thus it is difficult to effectively restore data. The FSTD is a scheme of distinguishing Tx antennas of the BS on a frequency basis in order to allow the UE to be able to estimate a wireless channel for each Tx antenna. In the FSTD, a frequency resource is allocated to each Tx antenna in a divided manner. There is a need for an apparatus and method for transmitting a synchronization signal capable of easily performing channel estimation for each Tx antenna when the synchronization signal is transmitted using a plurality of Tx antennas.